1. Field of the Invention
The present invention relates to a mixer, and more particularly, to a low noise mixer.
2. Description of the Prior Art
A mixer is an indispensable component for a wireless communication system. The mixer is mainly used for mixing a baseband signal and a local oscillator signal to generate an RF signal, or for mixing a RF signal and a local oscillator signal to generate a baseband signal. In U.S. Pat. No. 3,241,078, Gilbert discloses a conventional active mixer, now widely known as a Gilbert cell. Various improvements to the Gilbert cell are continuously proposed. The primary advantage of the Gilbert cell is the better signal gain at the output signal; however, the Gilbert cell has a serious problem with flicker noise. The flicker noise in a mixer or an oscillator may be transformed to phase noise thereby limiting the spectral purity.
One improvement to the Gilbert cell is a mixer with a common-mode feedback circuit, which is disclosed in U.S. Pat. No. 6,865,382. The common-mode feedback circuit can fix the DC voltage of the mixer output terminal at a predetermined working range. This kind of mixer can provide an adjustable gain. However, the common-mode circuit also generates flicker noise, such that the output signal of the mixer is disturbed. Another improvement to the Gilbert cell is a current-bleeding type mixer. The current-bleeding type mixer is disclosed in page 108, “5 GHz CMOS RF RECEIVER FRONT END CIRCUIT DESIGN” by Dr. Tang Gu Tsun, Department of Electrical Engineering, National Taiwan University, wherein the current-bleeding type mixer is used for decreasing the flicker noise come along with the Gilbert cell. Since the flicker noise is in proportion to the current passing through the switch circuit, the current-bleeding type mixer, therefore, provides an auxiliary current to decrease the current passing through the switch circuit, and as a result, alleviating the flicker noise. Additionally, different kinds of current-bleeding type mixers are further disclosed in U.S. Pat. No. 6,889,037, as well as in “Current-Reuse Bleeding Mixer,” Electronics Letters, 13Apr. 2000, Vol. 36, No. 8, IEE 2000 by Lee and Choi. Also, a different kind of mixer structure is disclosed in U.S. Pat. No. 6,850,753.